attempt at fixing click tracking

fixes [this bug](https://meta.discourse.org/t/firefox-middle-click-triggers-pop-up-blocker/7297).

i removed the `data-href` business, and replaced middle-click handling with the native browsers’. i also reduced code duplication.

dunno if this passes tests, let’s see if you have travis or something. once it passes and you insist, i could special-case for firefox.